A thermoformer or thermoforming machine typically has a series of stages that transform input plastic material into finished containers. The input material is a thermoplastic substance that can be formed when heated. A common thermoplastic material is polypropylene. The finished containers are commonly used to hold food or beverages, and may be formed into a variety of sizes and shapes depending on the thermoforming process that is utilized.
Thermoplastic materials may be fed into a thermoforming machine in the form of a continuous sheet or as individually cut blanks or billets. When the input materials are individual thermoplastic billets, the process is called “scrapless thermoforming” because the finished containers need not be cut from a sheet after forming, reducing the amount of scrap material. In scrapless thermoforming, a billet feeding unit is typically used to load individual billets onto a conveying device in the machine.
Prior to being formed into containers, the billets must be heated to the desired temperature. The desired temperature depends on the structure of the machine being used as well as the desired properties of the end product. For example, containers may be formed while the thermoplastic material is below the crystalline melt point of the material. Such forming is known as solid-phase pressure forming. Other methods involve heating the material to its melting point prior to forming. Such a process is known as melt-phase thermoforming.
A conventional scrapless thermoforming machine has several stages used to create formed containers. First, the billets are loaded into the machine. Second, an oven is used to heat the billets to the desired temperature. Third, a forming station or form press utilizes a hydraulic press or other suitable means to form the individual containers. After exiting the form press, the formed containers are removed from the machine at an unloading station. Other stations may be added to the thermoforming machine as desired, such as a pre-heating oven and a coining press to form an initial container edge prior to entry into the main oven.
One challenge associated with conventional thermoforming machines is the efficient use of the trays used to transport billets and associated tooling. A typical scrapless thermoforming machine has a billet loading station at one end of the machine and a container unloading station at the other end of the machine. Such a configuration requires the transport of unloaded empty trays from the container unloading station back to the loading station. One way of transporting the empty trays back to the loading station is to do so on a different vertical level from the loaded trays, such as by utilizing an elevator to move the trays onto a conveyor below the loaded trays for transport in the opposite direction. However, such a configuration may be disadvantageous because the long distance transport of empty trays is inefficient from an overall process standpoint, and the conveyor system required to transport the empty trays, such as the chains and belts and so forth, increases the number of components on the machine with a corresponding increase in the possibility of component breakage, thus increasing machine down time. Further, the machinery necessary to change the vertical level of the various trays in order to recycle the empty trays is complex, including drive mechanisms and tray transfer mechanisms, again increasing the chance of machine malfunction and down time.
An additional disadvantage of a thermoforming machine that requires empty tray transport over a substantial distance is the increase in the number of trays and amount of tooling required for machine operation. Each empty tray being transported at a given point in time represents an inefficient use of the tray, associated frames, and any clamps and so forth used to secure the billets.
Because scrapless thermoforming machines typically utilize a number of separate trays to transport plastic billets through the stages of the machine, a drive system is required to move the trays from station to station. A typical drive system includes a series of inter-connected chains or belts used to drive the transport trays. Design challenges associated with machines utilizing a chain drive system include designing the interface between the chains and the individual trays, especially where the trays change direction or vertical level, requiring a transfer from one chain drive to the next. Further, a chain drive system may not allow for individual control of trays, and therefore provides less flexibility in the movement of the trays, such as changes to tray speed or position, and may also be prone to malfunction, increasing machine down time.
Individual transport trays used in a scrapless thermoforming machine must have a mechanical interface with the drive system. A typical approach is to utilize wheels or rollers on the trays that ride within rails or another type of guide mechanism. One disadvantage of such a design is that the cost of each transport tray increases with the addition of the moving components, such as wheels.
Accordingly, there is a need for a scrapless thermoforming machine that makes more efficient use of tooling, including the transport trays used to convey plastic blanks. Further, there is a need for a scrapless thermoforming machine that does not utilize a chain or belt drive system to transport the trays from station to station in the machine. Further still, there is a need for a thermoforming machine that does not utilize wheels or rollers on each transport tray. It would be desirable to provide a machine and/or method that provides one or more of these or other advantageous features. Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the appended claims, regardless of whether they accomplish one or more of the aforementioned needs.